Using Kepler’s 3rd Law
Kepler’s third law: P2 㲍 a3
Therefore P2 = ka3, where k is some constant number.
We can find k if P is expressed in (Earth’s orbit) years and a
is expressed in terms of the distance between Sun & Earth.
This distance is called 1 Astronomical Unit, or 1 A.U.
Then substituting above values into P2 = ka3 we find:
(1)2 = k(1)3.
Solving for k, we find k = 1.
When these units for P and a are used:
P2 = a3 with P in years and a in A.U.
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